Various kinds of lighting systems for illuminating the harvester head are known from the prior art. In the known systems, the power supply to the work lights attached to the harvester head is arranged by bringing the input conductors from the base machine of the forestry machine along the set of booms and control has typically taken place from the cab of the forestry machine, either by active control by the driver or in practice by leaving the lights continuously switched on or off. However, the work lights often consume a great deal of power and can be controlled only as a totality. In some cases, the work lights can even make it more difficult for the operator to see the work clearly. In other words, it would be advantageous to implement the control of the work lights installed on the harvester head automatically, so that the driver need not think separately about controlling the work lights, never mind giving inputs to the control system for switching on the work lights or adjusting the properties of the illumination.